This application claims the priority of German Application No. 197 28 578.3, filed Jul. 4, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for controlling the evaporator temperature of an air conditioner as a function of the outside dew point. Here, the air temperature and dew point temperature of incoming air drawn in from outside by the air conditioner for air-conditioning an interior are determined. An evaporator demand temperature (without reheating) is determined from the setpoint information of the interior temperature.
The term "control" will be understood here in its broader sense and therefore includes, in addition to actual controls without feedback, regulation in which the effect of the control measure on the evaporator temperature itself or parameters influenced thereby (such as the temperature of the incoming air that is guided over the evaporator and blown into the interior, etc.) is detected by a sensor and fed back to a regulating unit. The evaporator demand temperature (without reheating) is determined from setpoint information of the interior temperature, and is the temperature value to which the evaporator temperature must be set when the incoming air blown into the interior is to be conditioned by the action of the evaporator alone(that is, without reheating, as noted). This is so that even the lowest setpoint value specified by the user in a single or multichannel air conditioner for the interior temperature in different interior areas is established without requiring reheating via a heating element connected downstream from the evaporator. This is in contrast to the case for evaporator temperatures that are lower than the demand temperature defined in this fashion.
Methods of this type are used especially in motor vehicle air conditioners. Here, essentially two operating modes are used during cooling operation. In a first operating mode, the evaporator temperature is set to the evaporator demand temperature (without reheating), as determined from the interior temperature setpoint information. This is so that no reheating is necessary and energy consumption is kept to a minimum. However, in many operating situations this results in undesired fogging of the windows. To prevent this, so-called reheating or counter-heating is provided as a second operating mode in which the evaporator temperature is set to a temperature value that is low enough to prevent fogging. This allows moisture to condenses out of the incoming air drawn in at the evaporator. Drier incoming air can be blown out into the interior as a result. Because, in this case, the evaporator temperature is below the evaporator demand temperature (without reheating) that corresponds to the interior temperature setpoint information, the reheating device for compensating for this temperature differential is activated. As a consequence, this operating mode is associated with higher energy consumption.
In a method known from German patent document No. DE 36 24 171 A1 for operating a heating and/or air conditioning system of a motor vehicle, the temperature and humidity of the interior air and/or of the incoming air drawn in from the outside are determined via corresponding temperature and moisture sensors. The temperature and humidity information are used to regulate the air conditioning of the interior. Under these conditions, via a temperature and moisture sensor pair located in the vicinity of the windows, the possibility that the dew point will be undershot at the inside surface of the vehicle windows is detected. An undershot is prevented by countermeasures that include increased ventilation of the inside of the window with incoming air drawn in from the outside. Another countermeasure (during recirculating operation) is performed with air drawn from the interior, with the air blown against the window being suitably conditioned by heating and/or dehumidification.
German patent document No. DE 37 24 430 A1 teaches a motor vehicle air conditioner which can be operated in the two operating modes, continuous cold regulation on the one hand, and reheating operation on the other. As long as the incoming air temperature is higher than the set interior temperature, interior air conditioning is performed by continuous cold regulation. Here, the air blown into the interior is regulated to the temperature at which the air blown into the interior can be held at a temperature that corresponds to the interior temperature setpoint information, without reheating being necessary. This is accomplished by suitably controlling the evaporator temperature by turning a compressor located in the cold circuit line of the evaporator on and off. If the incoming air temperature is between 0.degree. C. and the set interior temperature, air conditioning is performed in a moderate reheating operation in which the evaporator temperature is set to a variable setpoint that is established as a function of the incoming air temperature and the interior temperature setpoint information. Under these conditions, reheating takes place as a function of the deviation of the interior temperature from the setpoint and the evaporator temperature is set so that it is always below the incoming air temperature by a constant value that can be set to a specific value (6.degree. C., for example). The dew point temperatures of the incoming air and the interior air are not taken into account under these conditions.
It is known for example from German patent document No. DE 195 17 336 A1 to limit the setpoint of the evaporator temperature in a motor vehicle air conditioner for limiting odors to a presettable maximum value.
It is an object of the present invention to provide a method for controlling the evaporator temperature of an air conditioner as a function of the outside dew point, with which comparatively energy-saving interior air conditioning can be achieved. Simultaneously, the occurrence of disturbing condensation effects, especially precipitation on the outside surfaces of windows in the case of a motor vehicle air conditioning system can be avoided.
This and other objects and advantages are achieved by the method, according to the present invention, by controlling the evaporator temperature of an air conditioner as a function of the outside dew point, especially in a motor vehicle air conditioner, in which the air temperature and the dew point temperature of the incoming air drawn in from outside by the air conditioner for air conditioning an interior are determined and an evaporator demand temperature (without reheating) is determined from the interior temperature setpoint information. Here, the evaporator temperature is set to a setpoint that is chosen within presettable temperature limits as the smaller value of the evaporator demand temperature without reheating determined from the interior temperature setpoint information, on the one hand, and the difference between the air temperature and the dew point temperature of the incoming air blown into the interior and drawn in from the outside, on the other. With high outside humidity, the air temperature and dew point temperature of the incoming air are approximately the same so that the difference is approximately 0.degree. C., and hence is less than the evaporator demand temperature. Consequently, in this situation the evaporator temperature is likewise set to a setpoint close to 0.degree. C. The setpoint is thus advantageously limited at the lower end by a minimum icing protection temperature that can be set in advance in order to avoid undesired icing phenomena in the evaporator. Hence, the evaporator is set to maximum dehumidification when the outside humidity is high. As a result, when it is used in a motor vehicle air conditioner, for example, fogging of the windows is reliably prevented. If this does occur when the incoming air that has been drawn in is cooled too sharply for the desired interior temperature, this is compensated for by reheating.
When the outside humidity is low, the difference between the air temperature and the dew point temperature of the incoming air increases and the evaporator temperature setpoint is raised accordingly, so that less reheating power is required and energy is saved. As soon as this temperature differential reaches the value of the evaporator demand temperature corresponding to the lowest (possibly one of several) interior temperature setpoints provided in advance, the evaporator temperature setpoint is set to this demand temperature value and no reheating is required any longer.
In certain preferred embodiments of the present invention, the evaporator temperature setpoint is limited at the upper end by a presettable odor-avoiding maximum temperature. This maximum temperature is generally below the maximum temperature possible for the air conditioner itself. As a result, conditions are created such that excessive evaporator odors which are caused by the change between the dry and wet states of the evaporator surface do not develop.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.